Re: anyone ever done multicast AF_UNIX sockets?

Chris Friesen
2003-02-28T14:29:44Z

jamal wrote:
>
> On Thu, 27 Feb 2003, Chris Friesen wrote:

>>It is fairly common to want to distribute information between a single
>>sender and multiple receivers on a single box.

>>Multicast IP sockets are one possibility, but then you have additional
>>overhead in the IP stack.

> I think this is a _very weak_ reason.
> Without addressing any of your other arguements, can you describe what
> such painful overhead you are talking about? Did you do any measurements
> and under what circumstances are unix sockets vs say localhost bound
> udp sockets are different? I am not looking for hand waving reason of
> "but theres an IP stack".

From lmbench local communication tests:

This is a multiproc 1GHz G4
Host OS 2p/0K Pipe AF UDP RPC/ TCP RPC/ TCP
ctxsw UNIX UDP TCP conn
--------- ------------- ----- ----- ---- ----- ----- ----- ----- ----
pcary0z0. Linux 2.4.18- 0.600 3.756 6.58 10.2 26.4 13.8 36.9 599K
pcary0z0. Linux 2.4.18- 0.590 3.766 6.43 10.1 26.7 13.9 37.2 59.1

This is a 400MHz uniproc G4
Host OS 2p/0K Pipe AF UDP RPC/ TCP RPC/ TCP
ctxsw UNIX UDP TCP conn
--------- ------------- ----- ----- ---- ----- ----- ----- ----- ----
zcarm0pd. Linux 2.2.17- 1.710 9.888 21.3 26.4 59.4 43.0 105.4 146.
zcarm0pd. Linux 2.2.17- 1.740 9.866 22.2 26.3 60.4 43.1 106.7 147.

This is a 1.8GHz P4
Host OS 2p/0K Pipe AF UDP RPC/ TCP RPC/ TCP
ctxsw UNIX UDP TCP conn
--------- ------------- ----- ----- ---- ----- ----- ----- ----- ----
pcard0ks. Linux 2.4.18- 1.740 10.4 15.9 20.1 33.1 23.5 44.3 72.7
pcard0ks. Linux 2.4.18- 10.3 16.1 19.8 36.3 22.8 43.6 74.1
pcard0ks. Linux 2.4.18- 1.560 10.6 16.0 23.4 38.1 36.1 44.6 77.4

From these numbers, UDP has 18%-44% higher latency than AF_UNIX, with
the difference going up as the machine speed goes up.

Aside from that, IP multicast doesn't seem to work properly. I enabled
multicast on lo and disabled it on eth0, and a ping to 224.0.0.1 still
got responses from all the multicast-capable hosts on the network. From
userspace, multicast unix would be *simple* to use, as in totally
transparent.

The other reason why I would like to see this happen is that it just
makes *sense*, at least to me. We've got multicast IP, so multicast
unix for local machine access is a logical extension in my books.

Do we agree at least that some form of multicast is the logical solution
to the case of one sender/many listeners?

Thanks for your thoughts,

Chris

--
Chris Friesen | MailStop: 043/33/F10
Nortel Networks | work: (613) 765-0557
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